The present invention relates to optical disk recording devices for recording information on writable optical disks in the CD (Compact Disk) format. More particularly, the present invention relates to an improved optical recording device of such type which can accurately detect so-called "asymmetry" on the basis of reproduced level values of signals recorded on an optical disk and thus accurately determine an optimum recording power value, by being capable of effectively avoiding misdetection of the peak level values of the reproduced signals that would occur due to a defect such as scratch, dust or stain in or on the disk surface, or deficiency in the reflective surface of the disk.
When recording signals on a given recording medium such as a disk or a tape, so-called calibration may sometimes be performed, in which signals are first written, by way of trial, on a predetermined trial writing area of the medium before being actually recorded, then the trial signals are reproduced for examination of the signal quality, and an optimum value of writing energy or recording power to be used is determined on the basis of the signal quality thus examined. According to the known standards for write-once type Compact Disk (CD-WO), the trial writing area is provided on the innermost peripheral portion of the disk and is called a power calibration area (PCA), and a series of the above-noted operations is called optimum power control (OPC).
As an example, the optimum power control is performed in the following manner. First, test EFM (Eight to Fourteen Modulation) signals are written onto the power calibration area of an optical disk while the intensity value of recording laser power is varied either continuously or in a stepwise fashion. The thus-recorded test EFM signals on the power calibration area are then reproduced so as to detect, on the basis of the quality of the reproduced high-frequency EFM signals, a specific position on the calibration area where the EFM signals are written in an optimum condition. Thence, the value of the recording laser power with which the optimum EFM signals are recorded is determined as an optimum power value. The above-mentioned signal quality examination is carried out by detecting the asymmetry of the high-frequency EFM signals.
FIG. 2 shows in block diagram a typical circuitry arrangement conventionally employed for reproducing test EFM signals, previously recorded on an optical disk by use of sequentially varied recording power, so as to detect the asymmetry, and then determining an optimum recording power value on the basis of the detected asymmetry. More specifically, a reproducing laser beam is irradiated onto the power calibration area of the disk where the test EFM signals are recorded and the reflected light from the calibration area of the laser beam is received. High-frequency signals obtained as the received light signals are passed through a high-pass filter 10 which cuts off the direct current component contained in the signals. For each varied value of the recording power, top and bottom peak detection circuits 12 and 14 detect the top peak (positive peak) level At and bottom peak (negative peak) level Ab of each of the high-frequency signals, respectively, by analog processing. An asymmetry calculation circuit 16 calculates the asymmetry .beta. for each value of the recording power using an equation of .beta.=(At+Ab).div.(At-Ab). A determination circuit 18 finds out specific one of the recording power values which can achieve asymmetry that is closest to predetermined optimum asymmetry value (e.g., 0.04) and determines that specific recording power value as an optimum recording power value. By performing actual signal recording using the thus-determined optimum recording value, the best signal quality can be realized.
With the prior art arrangement of FIG. 2, each synchronizing signal of an 11T--11T signal pattern (e.g., a repetitive pattern, which is formed by a former section having a high level for 11T and a latter section having a low level for 11T) will normally have the greatest signal levels, and the top peak level At is detected of the former 11T section while the bottom peak level Ab is detected of the latter 11T section. However, defect such as scratch, dust or stain on the disk surface, or deficiency of the reflective surface of the disk, if any, would frequently be misdetected as a top or bottom peak value. Very often, this would prevent accurate detection of the asymmetry, and hence accurate determination of the optimum recording power value. In order to minimize the adverse influence of such peak value misdetection, it may be proposed to provide filters at the rear of the top and bottom peak detection circuits 12 and 14 in such a manner that the asymmetry calculation is performed after the detected peak values have been averaged. But, because the power calibration area is very narrow and short response time is required, for the asymmetry calculation, the peak value averaging period is considerably limited, so that the influence of the peak value misdetection could not be avoided completely.
Further, with some of the CD-WO disks, when reproducing a long mark (pit) of, say, 11T, there may be caused an appreciable waveform distortion near the trailing edge of the reproduced pit, as shown in FIG. 3. In such a case, the above-mentioned circuitry arrangement of FIG. 2 could not detect the asymmetry with high accuracy because the bottom peak level is detected at a projecting tip of the distorted waveform.